CliError/SkillInstallError carry every op-cli failure (94 sites),
ProgramError types batch_program (14), WebCanvasError maps route
failures onto their existing HTTP statuses (23 -> 6 deliberate
boundary adapters). Display output is byte-identical to the old
strings so caller-visible text and tests are unchanged.
The opencode target wrote a plugin entry the loader can never use (no
JS entrypoint, and plugin packages' skills/ are never scanned), so
'op install --target opencode' delivered nothing. Mirror the codex
layout instead: bundle under ~/.config/opencode/openpencil-skill with
a skills/ symlink opencode's {skill,skills}/**/SKILL.md scan picks up.
The installer-owned entry is recreated on every install so stale
squatters can't shadow it; remove_path only treats NotFound as absent
and handles dangling Windows directory symlinks.
